The sky condition of Singapore, a typical tropical location, is analysed using instantaneous screened data. Luminous efficacy relations for clear, overcast and partly cloudy sky conditions have been obtained and compared with those published for Garston, UK. The hourly average data for diffuse and global solar radiation and daylight illuminance have been analysed to obtain the average luminous efficacy values for an average year. The luminous efficacy relations for the different sky conditions can be used for detailed design studies, while for systems where an average performance assessment is adequate the use of the average efficacy values should suffice.